Hydrocarbon containing fuels such as gasoline are the most common power source for internal combustion engines. Gasoline must be carried by the vehicle, usually in a fuel tank. Heretofore fuel tanks have been manufactured from metal. More recently however, much work has been done on fuel tanks made from plastic resins, typically polyethylene. Polyethylene is a very suitable material for making fuel system components such as tanks in that it is readily moldable using blow molding techniques. However, it has been determined that fuel vapour can permeate through the wall of a fuel system component such as a fuel tank when the wall is manufactured solely from polyethylene. In order to provide suitable anti permeation characteristics, more complex wall structures for such fuel system components have been developed. In our co-pending patent application Ser. No. 09/192,295 filed Nov. 17, 1998, the disclosure which is herein incorporated by reference, there is a discussion of a multi-layer conduit. Such conduits are readily manufacturable using blow molding techniques.
In the blow molding technique, a parison is extruded from an extrusion head. The parison is normally allowed to hang vertically from the extrusion head as the correct amount of parison to make the desired part is extruded. The parison is placed between the open portions of a blow molding mold. The blow molding mold is then closed around the parison and the parison is pinched off at the top and bottom. A convenient structure, typically a blow molding needle, pierces the wall of the parison and blowing gas under pressure is introduced into the interior of the parison. The parison which at that stage is hot and still quite flowable is expanded outwardly by the blowing gas and the shape of the cavity in the blow mold determines the exterior configuration of the blow molded part. The blow molded part then has a wall thickness which is substantially uniform around the dimension of the part, subject only to certain thinning of the parison as it is stretched to meet the configuration of the mold cavity.
Plastic molded fuel tanks have now proven to be commercially acceptable on incorporation of some means to control permeation. Typically, the permeation can be controlled by barrier layers such as a layer of ethylene vinyl alcohol copolymer (EVOH) which is incorporated into a multi-layer parison and wall structure. Typically, in order to adhere the EVOH layer, adhesive is supplied to either side of the EVOH barrier layer as the barrier layer is extruded from the extrusion head. Typically, the adhesive attaches the EVOH layer to an outer layer of polyethylene and an inner layer of polyethylene. Either or both of the polyethylene layers may include either virgin material or scrap reground polyethylene material. Where required by the conditions, the inner layer of the fuel system component may also be modified so as to conduct electricity. This helps provide an electrical path to bleed off static electricity which might be generated in or around the fuel stored in the fuel system component. All of the various layers are simultaneously extruded through a multi channel extrusion head to produce a parison ready for molding.
While tanks of the type described generally above are proving quite satisfactory, there is a continuing need to further reduce the possibility of permeation of fuel vapours from fuel system components, in particular gasoline tanks. It has been discovered that the pinch off area which occurs in the molding operation may be a source of fuel permeation. This occurs because of breach or exposure of the barrier layer in the pinching process or in the subsequent process of flash removal.
Another area of fuel vapour permeation is through other fuel system components which may be attached to a tank such as flanges, pipe nipples, sender units flanges, fuel pump flanges and the like. When a flange is made from polyethylene or other material that is thermally fusible to the tank or fuel system component, that flange itself may provide a discontinuity in the barrier quality of the tank and then present a path for fuel vapour permeation.
Accordingly, there is a need to have a further improved fuel system component in which the component is better sealed to further inhibit fuel vapour permeation.